April, 1948
EFFECTSOF TEMPERATURE AND SOLVENT ON MONOMER REACTIVITY
2. Acetone and nitrosomethane were found to be initial products of the decomposition. The latter new compound was identified as the hitherto unknown crystalline dimer.
[CONTRIBUTION
1519
3 Some of the properties of nitrosomethane and its dimer have been determined. WILMINGTON, cALIFoRNIA RECEIVED SEPTEMBER 26, 1947
NO. 66 FROM THE GENERAL LABORATORIES OF THE UNITED
STATES RUBBER COMPANY]
Copolymerization. IV. Effects of Temperature and Solvents on Monomer Reactivity Ratios BYFREDERICK M. LEWIS.CHEVESWALLING. WILLIAM CUMMINGS.’ EMORENE R. BRIGGS~ AND FRANK R. MAYO Previous papers from this Laboratory arid elsewhere3 have shown that the behavior of monomers in free radical type copolymerizations may be described accurately by the copolymerization eq~ation~~,~,~
reactivity ratios are available. The results give partial support to Price’s suggestion and permit a much more detailed discussion of copolymerization phenomena than has hitherto been possible. The present paper discusses relinements in techniques and in the treatment of data which have been developed in this Laboratory during the past three years and presents measurements of where [MI] and [Ma] are concentrations of unre- the temperature coefficients of monomer reactivacted monomers, r1 is the ratio of the rate con- ity ratios for five monomer pairs. It also reports stants for the reaction of an MI-type radical with more precise measurements of the effects of solM1 and Mz, respectively, and r2 is the ratio for vents on the monomer reactivity ratios for styrene reaction of an M2-type radical with Mz and MI, and methyl methacrylate. respectively. The quantities r1 and r2 have been The next three papers, V-VII, describe new designated monomer reactivity ratios, and i t should experiments on twenty-nine monomer pairs. In be noted3bthat a comparisonof the reciprocals of a VIII, all of these data are reviewed and the theoseries of monomer reactivity ratios for a particular retical implications discussed in terms of monomer radical with a number of monomers yields the activity and polarity series. Copolymerization relative reactivities of the monamers toward that I X presents and discusses experiments on the radical. If such series for all radicals were the relative reactivities in copolymerization of cis same, i. e., if, in general, rlrz = 1, Equation (1) and trans isomers. The last three papers, X-XII, would reduce to the simpler form earlier proposed are a study of the effect of nuclear substitution on by Wall? However, a striking feature of free the reactivity of styrene in copolymerization. radical copolymerizations is that, in many pairs, Here, measurements on thirty-six systems throw each monomer prefers to react with the opposite further light on the nature of the “alternating eftype radical. This “alternating effect,” which fect” in copolymerization, can be discussed qualitatively in terms of rlrz products (r1r2being zero for complete alternation), Experimental appears to be an additional effect superimposed Materials.-Diethyl maleate and diethyl fumarate were upon a fundamental order of monomer reactiv- Eastman Kodak Co. materials, melting points -12 t o itySaband Price’ has suggested, on the basis of the -11 and 0 t o lo, respectively. They were used without purification. Styrene, methyl methacrylate and data available at the time, that i t arises from polar further methyl acrylate were commercial materials, fractionally interaction between radical and monomer. The distilled and stored in the ice-box until used. p-Chloropresent series of nine papers increases five-fold styrene was prepared by the decarboxylation of p-chlorothe number of monomer pairs for which monomer cinnamic acid. Its preparation and physical properties (1) Present address, Department of Chemistry, University of Minnesota, Minneapolis, Minn. (2) Present address, R. F. D. 2, Guilford, Conn. (3) (a) M a y o and Lewis, THISJOXJRNAL. 66, 1594 (1944); (b) Lewis, Mayo and Hulse, ibid., 67, 1701 (1945); (c) Bartlett and Nozaki, i b i d . , 68, 1495 (1946); (d) Alfrey, Goldberg and Hohenstein, ibid., 2464; (e) Fordyce and Chapin, ibid., 69,581 (1947). Further references will be found in these papers. (4) (a) Alfrey and Goldfinger, J . Chem. P h y s . , 12, 205 (1944); (b) Wall, THISJOURNAL, 66, 2050 (1944). (5) Hereafter in this series the new nomenclature for copolymerization constants, cf. Alfrey, Mayo and Wall, J. Polymer S a , 1, 581 (1946),is used. Thus, MI, Mz, r i and 7x correspond t o S, M u end p in previous papers. ( 6 ) Wall, ibid.. 63, 1862 (1941). (7) Price, J . Polymer Sci., 1, 83 (1946).
are described elsewhere.* Polymerization Technique.-Polymerizations were carried out in duplicate or triplicate on 1:4 and 4: 1 molar ratios of monomers in sealed tubes in absence of air, essentially as described in previous papers in this At 60°, 0.1 mole yobenzoyl peroxide was used as catalyst, a t 131”. no catalyst except for the styrene-methyl methacrylate system where 0.1% acetone peroxide was added. All polymers were soluble in benzene and were isolated by the frozen benzene technique.9 Results of all experiments reported here are listed in Tables I and 11. Experimental Errors.-Extensive experience in this laboratory has shown that, although any set of experiments may give a very small intersection in the graphical ( 8 ) Walling and Wolfstirn, THISJ O U R N A L , 69, 852 (1947). (9) Lewis and M a y o , Imi. Eng. Chem., A n a l . E d . . 17, 134 (1945).
1520 F. M. LEWIS,CHEVESWALLING, WILLIAMCUMMINGS, E. R. BRIGCSAND F. R. MAYO Vol. 70 TABLE I
TABLE I1
COPOLYMERIZATION EXPERIMENTS AT Two TEMPERATURES EFFECT OF SOLVENTS ON MONOMER REACTIVITY RATIOS OF Time, % Cin STYRENE (Mi) AND METHYLMETHACRYLATE (Mz) AT 60" [Miloa
63.21 39.77 16.04 63.24 39.62 15.75
[Mzloa Styrene 16.03 39.67 63.42 15.81 39.37 63.97
[Milo [MI]" hr. polymer (Mi)-Methyl Methacrylate (Mz) at 60°b 58.80 14.43 5.0 83.66 83.77 83.94 36.24 36.27 5.0 76.52 76.61 14.46 59.41 2.68 69.13 69.11 58.76 14.24 89' 83.98 84.00 35.88 35.78 8gS 76.70 76.53 12.83 56.39 68.5O 68.90 69.07
63.0 39.50 16.95
16.19 39.64 6.45
17.67 19.24 19.15 79.85 79.78 80.28
Styrene (Mi)-Methyl Acrylate (Md at 60" 78.65 14.28 73.30 2.8 71.55 71.49 71.59 80.88 15.66 75.25 3.7 71.55 71.67 71.68 80.98 15.37 74.98 3.7 71.59 71.61 71.73 20.21 74.12 18.68 3.7 85.68 85.63 85.69 20.18 74.72 18.85 3.7 85.79 85.72 85.62 21.03 75.62 19.74 3.7 85.48 85.39 85.46
19.65 19.64 22.32 79.80 80.30 78.80
80.92 80.85 81.20 20.17 20.00 20.81
63.51 64.05 16.13 16.05
Styrene (M3-Diethyl Maleate (Ma) at 60' 15.68 37.79' 36 89.81 89.71 14.54 15.62 38.77 14.48 36 89.63 89.76 62.77 100 76.75 76.74 7.985 59.12 62.69 8.030 59.31 100 76.60 76.57
63.88 63.53 65.68
15.87 15.95 15.85
15.93 15.72 15.88 64.99 63.83
Styrene (MI)-Diethyl Fumarate (Mn) at 60" 62.6 69.27 69.03 63.48 2.979 49.90 63.61 3.191 50.13 62.6 68.89 68.96 62.6 69.00 68.82 63.28 3.096 49.45 9.83 23 76.42 76.79 16.94 4.936 9.34 23 77.22 77.39 16.03 4.790
16.12 16.61 63.80 '63.85 9.615 40.09 39.57 10.14
64.22 63.81 16.34 16.10
46.3d 29.56 12.38
Same at 131O 10.73d 1.0 29.99 1.0 54.4 0.83
16.38 16.13 18.42 68.1Q 68.80 67.60
6.056 12.31 11.94
3.307 3.574 53.13 52.33
Same at 131" 75.28 .67 74.86 .67 75.00 .67 17.24 .67 17.17 .e7 17.94 .67
Same at 131' 10.59 66 12.45 18 11.89 18
Same at 131' 45.22 219.3 44.81 219.3 12.16 2.0 11.76 2.0
84.45 84.34 76.59 76.42 68.39 68.41
70.69 70.77 70.84 70.94 70.93 70.88 71.67 71.68 71.65 86.03 85.99 86.04 86.09 86.18 86.11 85.92 85.87 85.90
87.50 87.42 88.62 88.47 88.22 88.35
66.30 66.50 78.02 78.20
66.43 66.59 78.07 78.30
Styrene (MI)-p-Chlorostyrene (MI) at 60' 32.27 6.825 22.10 14 21.18/ 21.18 10.06 31.33 7.32 14 7.48f 7.51 12 7.87 7.78 10.96 29.97 7.77 41.61 7.555 30.47 12 21.70f 21.78
39.70 10.34 10.07 39.90 10.09 39.55 40.00 10.15 10.21 39.90
Same at 131' 5.86 21.80 23.90 4.30 3.80 13.37
24.73 5.78 6.34 19.29
1.75 1.25 1.25 1.50 1.0
7.21f 7.41 21.66f 21.71 21.632 21.74 6.90f7.08 21.56f 21.68
a Millimo1e:s of unreacted monomers ; zero subscripts indicate initial quantities. Data taken from Mayo and Lewis, ref. 3. experiments 4B, 4C, 4D, 5A, 5B, 5C listed in that order but recalculated using empirical analyses on blanks (4A and 4E) for calculating polymer composition (see text). c Calculated using 92.24 as yo C in styrene (see text). Calculated using same blanks as 60 O experiments. Thermal polymerization, no catalyst added. Per cent. C1 in polymer.
[Milo
[Mzlo
[Mil
[Mzl
Time,
hr.
% c in polymer
Benzene (9 vol.)" 63.7 16.01 51.7 11.91 72 84.40 84.39 84.10 39.8 40.02 29.9 30.6 72 77.11 76.75 76.96 16.03 63.79 8.37 43.79 72 69.13 69.34 69.20 Acetonitrile (8 vol.)" 63.06 17.07 55.74 14.27 72 83.74 83.43 46.12 39.84 32.87 32.77 72 76.61 76.69 16.13 71.70 9 . 5 5 52.47 72 68.49 68.45
63.44 40.60 16.03 Per
19.07 39.54 66.11 volume
Methanol (2 vol.)" 55.98 16.13 24 83.30 83.43 32.33 32.12 24 77.31 77.29 7.09 42.45 24 69.14 69.05 total monomers.
solution of the copolymerization equation, these intersections shift appreciably from set t o set. In early work many of these shifts proved t o be due t o inadequate techniques of polymer isolation. With more refined methods, they now appear t o be usually the result of small systematic errors in polymer analysis. The temperature coefficients of monomer reactivity ratios discussed in this paper represent, a t best, small differences between experimentally measured quantities. Accordingly, the highest attainable accuracy in determination of monomer reactivity ratios is important. Further, since the desired quantities are differences, the presence of a small systematic error (so long as it is the same for the experiments at'poth temperatures) causes no trouble. In order to freeze" this error as nearly as possible, each set of experiments at two temperatures reported here was carried out and worked up by the same operator at the same time and using the same technitues. The relative experimental error at each temperature was then calculated as the standard deviation of duplicate experiments, and the errors in heats and entropies of activation were determined by the usual formulas for propagation of error. In the most fortunate cases, for example, styrene-methyl acrylate, the relative experimental error in monomer reactivity ratios determined in this way is considerably smaller than the probable absolute error. However, for the reasons outlined above, we consider it the proper one to use in the subsequent calculations. The styrene-methyl methacrylate system was studied before this procedure was adopted. The experimental error in this case was taken as that arising from a 0.1% error in carbon analysis (see below). An idea of the agreement obtained between experiments and the magnitude of the change in monomer reactivity ratios arising from a 70" change in temperature may be gotten from Fig. 1 in which the graphical solutions of the copolymerization equation for the styrene-methyl acrylate system are illustrated. In the case of the styrene-diethyl maleate system, the monomer reactivity ratio for the maleate-type radical is indistinguishable from zero. Accordingly, only 4 :> styrene-maleate experiments were carried out a t 131 , and the heats and entropies of activation differences for the reaction of the styrene type radical calculated from the shift of the intersection of the high styrene experiments with the zero axis. It is of course important t o have an idea, as well, of the magnitude of the absolute experimental error in the measurement of monomer reactivity ratios. Since, with suitable technique in polymer isolation, this error arises chiefly from errors in polymer analysis, we have adopted the following technique for its estimation, based upon the observation that blank carbon analyses run in
April, 1945
EFFECTS O F TEMPERATURE AND SOLVENT ON
our analytical laboratory on known samples deviate from calculated results by over 0.2% less than one time in five. For a given monomer pair two representative experiments, one a t 1 :4 and the other a t 4: 1 monomer ratios are chosen and [Ml] and [Mz] recalculated assuming +O.Z% and -0.2% errors in analysis. The results are then replotted on a rl us. r~ plot, yielding a parallelogram about the former intersection. This parallelogram is now shifted so that ita center coincides with the best value of rl and r2 determined by the whole set of experiments carried out on the given monomer pair, and the absolute experimental error is taken as the range of values of 11 and r2 lying within the parallelogram. The best justification for the method, aside from the arguments just outlined, is that with sufficient care sets of experiments can generally be obtained in which all of the lines corresponding t o the individual experiments pass through the parallelogram. In the case that they do not, and it does not appear feasible t o run additional polymerizations, the standard deviation is taken as the absolute experimental error. However, this difficulty has not arisen with any of the monomer pairs reported here. In the cases where [MI] and [M*] are determined by nitrogen or chlorine ’ experimental analysis, calculations are based upon a 0.1 % error. In the case of the system styrene-methyl methacrylate, blanks of pure polymeric styrene and methyl methacrylate were analyzed simultaneously with the copolymers and the empirical carbon contents used in calculating copolymer compositions. Although the experiments are those described in the f i s t paper of this series,” this changes the values of the momomer reactivity ratios slightly from the values previously reported.l0 For styrene-diethyl maleate, polymer compositions lay very close t o pure polystyrene. Accordingly, a sample of pure polystyrene t o which diethyl maleate monomer had been added was worked up together with the copolymers and analyzed. The result (92.24% C) was used in calculations and served as a check both on the isolation procedure and the accuracy of carbon analysis (calcd. 92.2670). In view of the employment of these precautions, the use of 0.1% error in carbon seemed justified in calculating the experimental errors in this system.
MONOMER
REACTIVITY
1521
0.f
r;0.5 u h
t 3
v
:
0.7
0.2 0.3 (Methyl acrylate). Fig. 1.-Copolymerization of styrene and methyl acrylate at 60” and 131’. Numbers of lines correspond t o order of experiments in Table I. 0.1
YZ
simpler way of visualizing the magnitude of a “steric” effect, it has been calculated as well, and is included in Table 111. Two conclusions may be safely drawn from the data of Table 111. The first is that, despite the care with which the experiments were carried out, the resulting uncertainties in heat and entropy of Results and Discussion activation differences are still quite large. The Measurements at Two Temperatures.-Previsecond is that, for most of the pairs, entropy of acous measurements of monomer reactivity ratios tivation differences do not differ significantly from in copolymerizations have been limited to a zero, and the major source of the differences in single temperature. Measurement at two tem- reactivity of monomers in polymerization lies in peratures are of obvious practical interest. heats of activation. Accordingly, the practice of Further, since a monomer reactivity ratio repre- discussing these differences as due to resonance sents the ratio of two rate constants which may stabilization of complexes, polar interaction, etc., is be expressed in the form in general justified. The only case where the entropy difference clearly differs from zero is that of the reaction of the fumarate type radical with styrene and diethyl fumarate. Although the where AS2 AH?, A S Z and AH: are, respec- reason for the difference, corresponding to a threetively, the entropies and heats of activation for fold difference in PZ factors, cannot be stated the reaction of MI type radical with M Iand Mz, unequivocally, it lies in the direction which would measurement of rl a t two temperatures permits be expected if the second carboethoxy group the calculation of the differences in the heats and of diethyl fumarate offered steric hindrance toentropies of activation for the two reactions of the ward the attack of the radical on the double bond. radical. These differences, for the monomer pairs If!)ht were the case, a similar difference should be of Table I, are listed in Table 111. Since ( A S - anticipated for the styrene radical in the same copolymerization and in the system styrene-diethyl AS$)/R = In (P11Z11/P&p) in the Arrhenius maleate. Although in both cases the entropy diftreatment, and since this ratio perhaps provides a ference does lie in the right direction, it is smaller (IO) Although blanks were run indicating slightly low (0.1-0.3%) and, in the first, within experimental error of zero. carbon analyses, the original calculations were based on theoretical Another result of these measurements is that carbon analyses. We have since frequently based calculations on in general the temperature coefficients of the actual instead of theoretical analyses and this procedure IS n o w exmonomer reactivity ratios are rather small, This tendrd t o our earlievwork; cf. Nozaki, ref. 11,
2
WILLIAMCUMMINGS, E. R. BRIGGSAND F. R. MAYO Val. 70 1522 F. M. LEWIS,CHEVESWALLING, HEATAND
TABLE I11 ENTROPY OF ACTIVATION DIFFERENCES IN THE COPOLYMERIZATION OF SOME MONOMER PAIRS YI
60'
Radical type5
$1
+
AHII - AHir
131'
Styrene 0.520 t 0 . 0 2 6 0.590 *0.026 Methyl methacrylate .460 * .026 ,536 * ,026 Styrene .747 * .028 .825 * .005 Methyl ,acrylate .182 * ,016 .238 * ,005 Styrene 6.52 * .05 5.48 * .56 Diethyl maleate < .Ol ............. Styrene .301 * .024 .400 * .014 Diethyl fumarate .0697 * .0041 .0905 * .0008 Styrene .742 * .030 ,816 * .015 p-Chlorostyrene 1.032 * .030 1.042 * .015 0 Each monomer of the pair being considered as MI in turn.
480 * 250 580 * 280 380 * 140 1020 * 340 -660 * 480
..... 1070 * 320
990 * 290 360 * 170 35 * 120
+
+ - ASn 0.12 * 0.68 .19 * .76 .54 * .36 .66 * .86 1.87 * 1.36 AS11
..
.82 * -2.35 * .48 * .40 *
.82 .73 .43 .32
PlIZ11/PI2Z12
1.06 * 0.30 l . l O * .34 1.31 * .16 1 . 3 9 * .49 2.55 =t1.26
. .. . ...
1.50* 0.31 * 1.27 * 1.22*
.50 .14 .24 .18
is, indeed, a necessary consequence from the ob- tric constant, respectively) and in methanol (a served small differences in entropies of activation, solvent from which the polymer precipitates) are and may be generalized to the statement that listed in Table I1 and the graphical solutions ilthe composition of the copolymer obtained from lustrated in Fig. 2. Since experiments were carsystems in which neither monomer reactivity ratio ried out using better techniques than in the first differs grea.tly from unity will be quite insensitive paper, intersections are smaller and render more ~ J ~solvents have no to temperature. This conclusion is of some prac- certain the c ~ n c l u s i o n ~that detectable effect on monomer reactivity ratios. tical importance. Absolute Values of Monomer Reactivity This conclusion is also in agreement with the Ratios.-Using the procedure described in the results of other workers who have found identical Experimental Part, absolute experimental errors monomer reactivity ratios for the systems styfor monomer reactivity ratios for the monomer rene-methyl methacrylate12 and styrene-acrylopairs studied here have been calculated, and re- nitrile3= studied under homogeneous conditions sults are summarized in Table IV, together with and in emulsion. the results of other workers on the same systems. The agreement between work in this Laboratory and elsewhere will be seen to be quite satisfactory. TABLE IV MONOMER REACTIVITY RATIOS AT 60' WITH CALCULATED EXPERIMENTAL ERRORS Error as-
Radical typt: Styrene Methyl methacrylate Styrene Methylacrylate Styrene Diethyl maleate Styrene Diethyl fumarate Styrene p-Chlorostyrenc
sumed, Monomer reactivity ratios '?lo This paper Other workers 0 . 2 C 0.520 * 0.026 (60') 0.65 *0.08" .2 C .460 * .2 C .75 * .2 C .I8 * .1 C 6.52 * .I C ,005 * .2 C .30 * .070 * .2 C .1 c1 .74 * 1 CI 1.025 *
.
.026 .07 .02 .50 .01 .02 ,007
.03 .05
* .IOa .76(.t0.1)b (70') . 2 (*0.05)b (70') 5 (*1.5)b (70°) 0 (*O.l)b (60')
.51
(70')
...
... c c
Nozaki, J . Polymer Sci., 1, 455 (1946). Results of two sets of experiments have been averaged. bAlfrey, Merz and Mark, ibid., p. 37. The experimental errors have been estimated by plotting their data on a ~1 IS. re plot and by taking the axes of the smallest ellipses through which all the lines corresponding t o their experiments would pass. "Marvel and Schertz, THISJOURNAL, 65, 2054 (1943), prepared and analyzed samples of this copolymer but monomer compositions were not varied sufficiently for a calculation of monomer reactivity ratios. (I
Effect of Solvents.-Copolymerizations of styrene with methyl methacrylate in benzene and acetonitrile (solvents of low and high dielec-
0.0 0.0
0.2 0.4 0.6 0.8 1.0 (Methyl methacrylate). Fig. 2.-Constancy of monomer reactivity ratios for styrene-methyl methacrylate in various solvents at 60': 47-49, benzene; 50-52 acetonitrile; 53-55, methanol. Numbers of line correspond t o order of experiments in Table 11. Black triangle represents intersection in absence of solvent. ~2
Acknowledgment.-The inception of work on copolymerization in these laboratories is largely due to the early decision by Dr. Robert T. (11) Nonaki, J , Polymer Sci., 1, 455 (1946). (12) Smith, THIS JOURNAL, 68, 2069 (1946).
COPOLYMERIZATIONS OF VINYLACETATE
April, 1948
Armstrong that a study of copolymerization would be one of the best approaches to a fundamental understanding of polymerization as a whole, a decision which we feel has been amply justified. We are also indebted to Dr. Oscar W. Lundstedt who was in charge of analytical work while most of the work in this series of papers was in progress. The consistency of analytical results discussed in this paper is largely the result of his efforts. Finally, we wish to acknowledge the considerable contributions of Mrs. Charles J. Pennino and Miss Lucille Librizzi, who have carried out much of the actual experimental work described in this series.
Summary 1. By carrying out copolymerizations at 60
[CONTRIBUTIONNO. 67 FROM THE
1523
and 131°, heat and entropy of activation differences for the reaction of each radical with the two monomers have been determined for the systems styrene-methyl methacrylate, styrene-methyl acrylate, styrene-diethyl maleate, styrene-diethyl fumarate and styrene-p-chlorostyrene. 2. In every case the difference in reactivity of the two monomers is found to be due, primarily, to differences in heat of activation. Only in the reaction of the diethyl fumarate radical with styrene does the difference in entropies of activation clearly differ from zero by more than experimental error. 3. Further data are presented showing that solvents (benzene, acetonitrile or methanol) are without effect on the monomer reactivity ratios of the system styrene-methyl methacrylate. PASSAIC,
NEWJERSEY
RECEIVED JULY 17, 1947
GENERAL LABORATORIES OF THE UNITED STATES RUBBERCOMPANY]
Copolymerization. V.' Some Copolymerizations of Vinyl Acetate BY FRANK R. MAYO,CHEWSWALLING, FREDERICK M. LEWISAND W. F. HULSE~
This paper presents experiments on the copolymerization of vinyl acetate with eight representative monomers. The double bond of vinyl acetate proves to be one of the least reactive of any common monomers toward free radical attack.
Experimental Materials.-Vinyl
bromide was prepared from ethylene bromide by the action of alcoholic sodium hydroxide. After washing with water and drying with potassium carbonate, the fraction used boiled a t 15.5-16.0' at 761 mm. Vinyl chloride, obtained from the Dow Chemical Co., was used without purification. The other monomers were commercial materials fractionally distilled before use and stored in a refrigerator. Procedure.-With the exceptions noted below, reaction mixtures were prepared as described previously' and products were isolated by the frozen benzene technique.' In the trichloroethylene experiments (5.00 g. of total monomers and 6.1 mg. of benzoyl peroxide) air was displaced from the reaction tubes by flushing with nitrogen and the polymers obtained by distilling off the monomers and heating the residue for sixteen hours a t 90-100' and 2 mm. pressure. The acrylonitrile runs were carried out in the presence of 5 cc. of acetonitrile. The low nitrile runs remained homogeneous. These polymers were precipitated twice from acetone solution with petroleum ether and were then pressed out into thin sheets and dried for twenty hours a t 60' and 1 mm. pressure. The high acrylonitrile runs gave a very fine suspension of polymer which at ikst gave no indications of coagulating or settling. As soon as these indications appeared, heating was stopped. The mixtures were diluted with benzene and petroleum ether; the polymer was collected on a filter as a white powder, washed with the latter solvent and dried for twenty hours a t 60' and 1 mm. pressure. The vinyl halides were measured out approximately by volume; their exact weights were determined by dif(1) For the preceding paper in this series, see Lewis, Walling, JOURNAL, 70, 1519 (1948). Cummings, Briggs and Mayo, THIS (2) Present address, Department of Geology, Bureau of Mineral Research, Rutgers University, New Brunswick, N. J. (a Lewis and Mayo, Ind. En;. Chem., Anal. E d . , 17, 134 (1945).
ference from the weights of the total contents of the reaction tubes. Copolymers containing large proportions of vinyl halide were insoluble in the reaction mixture (except when chlorobenzene was used as solvent) and in benzene. The excess vinyl halide was allowed t o escape and the polymers were precipitated twice from chloroform (bromide) or a chloroform-acetone mixture (chloride) and petroleum ether. The chloride polymers were broken up and heated for about twenty-four hours at 60' and 1 mm. pressure. Solvent was removed from the bromide polymers by twenty-four hours of evacuation a t 0" and 1 mm. pressure. They were finally warmed cautiously for a few minutes in warm water. Longer or stronger heating led to very rapid discoloration. Analyses for acetic acid4 were carried out by determining hydrolyzable acetoxy groups as acetic acid. The polymer sample (0.3-0.8g.) was weighed into a flask, dissolved in 30 ml. of benzene, and treated for forty-eight hours a t room temperature with 50 ml. of 0.5 N alcoholic sodium hydroxide. Benzene and alcohol were then removed by steam distillation, adjusting heat and steam input t o maintain about the same volume of solution. The mixture was next acidified with 15 ml. of phosphoric acid and 500 ml. of steam distillate collected. The steam distillate was gently aerated for twelve minutes to remove carbon dioxide and titrated t o phenolphthalein end-point using decinormal sodium hydroxide. A blank correction (-0.3 ml.) was applied and the results calculated as per cent. acetic acid in the original polymer sample. Data on experiments are summarized in Table I . In the copolymerization with vinyl ethyl ether, the monomer reactivity ratio for the ether was assumed t o be zero and the vinyl acetate monomer reactivity ratio calculated from two duplicate experiments.
Discussion Monomer reactivity ratios obtained from the data of Table I are summarized in Table 11. Since data on the eight systems were gathered a t scattered times over four years, during which analytical precision has varied, the standard deviation (4) The authors are indebted t o Dr. Ellen Bevilacqua for the development of the analytical method described, and also for most of the analyses reported here.
